The demand for wireless data services, such as mobile Internet, video streaming, and voice over IP (VoIP), have led to the development of high speed packet data channels to provide high data rates needed for such services. High speed packet data channels are employed on the forward link in IS-2000 (also known as IxEV-DV), IS-856 (also known as IxEV-DO), and Wideband Code Division Multiple Access (WCDMA) systems. The high speed packet data channel is a time shared channel. Downlink transmissions, (where “downlink” is a signal received by a subscriber radio device from a Base Transceiver Station (BTS) or base station), are time-multiplexed and transmitted at full power.
At any given time, a base station may transmit a packet to one or more mobile stations on the physical layer channel known as the downlink high speed packet data channel. Deciding which mobile station(s) to serve with the packet at a given time is the function of a “scheduler.” A number of different scheduling strategies can be used, each with a different implication for system throughput and fairness. Typical scheduling strategies employed include round-robin, maximum throughput, and proportional fairness. In addition, quality of service requirements frequently add scheduling complexities. For example, VoIP packet data, due to its conversational characteristic, typically has a relatively short maximum allowed transmission delay before service is considered to have degraded unacceptably. Thus, it is commonly necessary to schedule VoIP packet data for transmission more frequently than other packet data in order to maintain acceptable service.
However, simply scheduling VoIP packet data more frequently may result in inefficient use of available resources. This is because VoIP data is typically supplied at a relatively low rate. As a result, VoIP data queued for transmission to a particular mobile station is typically less than a full packet's worth of data. Thus, if only that VoIP packet data is transmitted in a given physical layer packet, the packet is most probably less than full, and typically considerably less than full. Transmitting less than full packets, particularly relatively lightly loaded packets, unnecessarily consumes available system resources, and may result in a degradation of the service provided to the other mobile stations being served by a given base station.
International publication no. WO 2006/055173 discloses a method for scheduling use of a downlink packet data traffic channel shared by a plurality of mobile stations. The method comprises the steps of calculating a ranking metric (or scheduling priority) for a mobile station that varies directly with the mobile station's scheduling downlink transmission rate and a delay factor indicative of the staleness of data queued for the mobile station, and scheduling one or more downlink transmissions to the mobile station on the downlink packet data traffic channel based on said ranking metric. Such a method is better adapted to the downlink transmissions of VoIP data on high speed packet data channels.
The delay based scheduling described in the above-referenced international publication is however only used in the downlink. In a wireless communication system, some types of signaling traffic such as SIP (Session Initiation Protocol) traffic includes uplink (where “uplink” is the signal sent from a subscriber radio device to the base station) and downlink traffic being transmitted concurrently. Generally, the uplink traffic includes the request messages or acknowledgement or response needed to be transmitted to a SIP server, and the downlink traffic includes the response messages or acknowledgement needed to be transmitted to an SIP client. Uplink traffic may therefore interact and interfere with downlink traffic and the resulting performance loss in uplink traffic and/or downlink traffic will degrade the overall performance of the communication system. For example, if one uplink message is delayed due to the system overload in the uplink, a SIP setup phase duration will be increased. This means that the number of concurrently served SIP sessions will be increased (assuming that the SIP session arrival follows the Poisson process) and the uplink noise rise and the load on the uplink will consequently be increased. This will decrease the system capacity and increase the SIP setup delay. The same applies to downlink messages.